1. Field of Invention
This invention relates to a programmable surface acoustic wave (SAW) tapped delay line, and more particularly to a nonlinear programmable SAW delay line providing selective bi-phase programming of the individual tap electrodes.
2. Description of the Prior Art
The surface acoustic wave (SAW) tapped delay line is well known in the art, and the use of these SAW devices for encoding and correlating fixed and programmable phase shift key (PSK) codes, and for providing nondispersive and dispersive filters for generation and detection of various waveforms is also known, as reported by M. G. Holland and L. T. Claiborne, "Practical Surface Acoustic Wave Device", Proceedings of the IEEE, Vol. 62, pages 582-611, May 1974. The basic function of the SAW device in both applications is to provide the real time signal correlation and convolution of input signals.
The use of SAW delay line devices for real time signal correlation and convolution of input signals through nonlinear wave interaction is described by C. F. Quate and R. B. Thompson, "Convolution and Correlation in Real Time With Nonlinear Acoustics", Applied Physics Letters, Vol. 16, pages 494-496, June 1970. The nonlinear SAW correlator and convolver disclosed by Quate and Thompson provides the convolution and correlation of two applied signals by taking the product of the two signal frequencies, and integrating the product over a period of time. The product of the two signals in the Quate convolver is achieved by the nonlinear interaction of the two amplitude modulated SAW's as they pass through a piezoelectric crystal with different phase velocities causing an electrical polarization to be induced in the piezoelectric crystal which is proportional to the product of the two amplitude modulated functions. In a commonly owned U.S. Patent of the same assignee entitled "Diode Coupled Tapped Acoustic Delay Line Correlator and Convolver", U.S. Pat. No. 4,016,514, filed Oct. 9, 1973, a diodetapped delay line correlator is disclosed which is similar in operation to the nonlinear acoustic wave correlator disclosed by Quate and Thompson, but which in contrast provides the nonlinear signal frequency mixing in the diode associated with each tap, the nonlinear, signal transfer characteristic of the forward biased diode providing the product of the applied input signal. The large nonlinearity available in the semiconductor diodes provides a greatly enhanced product signal amplitude, with a correspondingly low insertion loss, however, only uniphase operation of each tap is permitted.
Since the significant uses of the SAW tapped delay line in the communication field relate to correlation and encoding of (PSK) signals which require bi-phase operation of the taps, the development of a SAW delay line apparatus which provides nonlinear wave interaction and which can be readily programmed for bi-phase operation is desirable. Bi-phase SAW devices have been developed for switching the taps to one of two phases which are displaced by 180.degree., through the use of a quaddiode switching technique, as described by Holland and Claiborne, Ibid., at pages 596, et seq. The quad-diode switching configuration therediscussed, and similar such devices presently known in the art function as electronic single pole, double throw switches which selectively reverse the opposite phase electrodes of the individual delay line taps to provide the bi-phase operation. These devices suffer from a number of disadvantages including: complexity, a relatively large number of components which results in low reliability, high DC power requirements, and a low switching speed resulting from the combined diode junction capacitance.
Similarly, the use of the SAW tapped delay line in providing programmable dispersive, or nondispersive transversal filters, requires a SAW device capable of providing nonlinear wave interaction together with both bi-phase operation of the taps and selectively adjustable tap signal amplitudes, to provide a programmable phase/amplitude device suitable for frequency filter synthesis. In both applications, that of bi-phase operation of the delay line taps, and bi-phase operation of the taps combined with adjustable tap signal amplitudes, high programming speeds and low power consumption are required to provide high reliability devices. Such programmable SAW delay lines are not presently known in the art.